Hemodialysis treatments have been developed since the early 1960s using a variety of combinations and arrangements of catheters. The earliest treatments were conducted using two needles in the same vein, and this subsequently led to pioneer work done by Dr. Shaldon in England who used two flexible catheters which could be left in place for limited periods. It was recognized by some practitioners that it would be preferable to use a single incision rather than to use two incisions and this led to the development of techniques involving dual flow catheters. There are two basic types. The first to be attempted was a coaxial catheter with the intake lumen surrounding the return lumen. While this had advantages, there were some difficulties of manufacture. The other approach is to use side-by-side lumens either in individual tubes connected to one another or in a single tube divided by an interior septum so that the lumens are D-shaped. These structures also had advantages and disadvantages, the notable disadvantages being that because the lumens are side-by-side, the intake openings must be in one side of the catheter. As a consequence of this, if the catheter were to attach itself to the wall of a vein due to suction applied to the intake lumen, then the flow would stop. Medical staff then have to move the catheter by rotating it until blood again flows. This is a very delicate manipulation which is normally performed only by a qualified medical practitioner who must be available at all times in case the flow is discontinued.
The side-by-side structures have advantages in manufacture due to the fact that the two lumens can be created simultaneously in an extrusion. This has led to great activity in developing devices having side-by-side D-shaped lumens at the expense of coaxial structures. Nevertheless, due to the inherent disadvantages of the side-by-side structures, there has been renewed interest in developing suitable coaxial devices. This is primarily because the intake lumen can have openings in any part of the wall of the catheter. As a result, no matter where the catheter may rest against a vein, some of the intake openings remain patent. There is then less likelihood that the procedures must be serviced by a trained medical practitioner.
An early patent showing the use of coaxial tubing in haemodialysis treatments is U.S. Pat. No. 4,037,599 to Raulerson. This structure involves the use of a needle forming the return lumen and a coaxial sleeve which is tapered at its leading end to follow the incision made by the needle and to provide an intake lumen between the tube and the needle. This structure can not of course be left in place, but it is an indication of the approach to the use of coaxial tubes to form two lumens.
Another structure involving the use of a needle is found in U.S. Pat. No. 4,073,297 to Kopp. Again this structure involves a tube about a needle to define an intake lumen and blood is returned through the needle.
A further approach to co-axial dual lumens is found in U.S. Pat. No. 4,196,860 to McLaughlin. In this structure the intake lumen is open at its end, and blood is withdrawn down this lumen about the return lumen. Further structures of a similar kind are to be found in U.S. Pat. No. 4,202,332 to Tersteegen et al.
U.K. Patent Application No. GB 2017499 A teaches the use of a dual lumen coaxial catheter made of flexible plastics material. The inner tube projects beyond the outer tube to form a return lumen, and the intake lumen is tapered at its end to close about the return lumen. This kind of structure is also to be found in Canadian Patent 1,150,122 to the present inventor.
A contrasting structure is shown in U.S. Pat. No. 4,493,696 to Uldall. In this the outer tube converges to fit about the inner tube and then projects beyond the inner tube to form an extension of the inner tube at the tip. Although this contrasts with all of the previous structures mentioned, there are disadvantages in this arrangement. Notably that there is a discontinuity where the end of the inner tube terminates, and the rigidity of the tip is determined by the material of the outer tube, which of course must be sufficiently rigid to maintain its patency in use.
Another reference known to applicant and involving a coaxial dual lumen arrangement is found in U.S. Pat. No. 4,666,426 to Aigner, which uses the inner tube to form a tip.
These prior art structures suffer from contrasting design criteria which have the effect of limiting the acceptance of coaxial dual lumen catheters in hemodialysis. In all instances the tip section is made up of material formed from an extension of either the inner or outer tubes forming the dual flow portion of the catheter. As a result the tip section has the same physical characteristics as parts chosen to be sufficiently stiff to resist kinking, if the catheter should be bent. By contrast, it is desirable that the tip section be sufficiently soft and pliable to permit this section to take up the local shape of a vein containing the catheter, thereby to avoid applying distorting forces to the vein and to permit prolonged access with a suitable selection of materials.